Having two partitions is just good practise in general; you have a small (a couple dozen megs is more than enough for 3.1 at least) boot partition that holds Workbench and maybe some core utilities, but not much else, and then you have a separate work partition for applications and games (a third one for data is also common).

This lessens the likelyhood of a game or other software somehow corrupting the drive and preventing you from booting the system. Of course, it's in no way necessary and you can have everything on the same partition if you like.